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Using the Boiling Point of Water to Learn about Hydrogen Bonds

quick fix Using the Boiling Point of Water to Learn about Hydrogen Bonds SuSan Offner AbstrAct The boiling point of water is 300°C higher than expected because of hydrogen bonds. Key Words: Water; hydrogen bonds; boiling point. I then fill in the boiling points as follows. I round to the nearest whole number, but here I give the actual numbers for reference. Absolute zero is –273°C, or 0°Kelvin. H2 = –252.87°C N2 = –195.79°C The boiling point of water can be used to illustrate the importance of O2 = –182.95°C hydrogen bonds in living systems. The boiling point of a compound is the temperature at which a liquid becomes CO2 = –78.5°C a gas. All other things being equal, the boiling The boiling point of point should be approximately proportional The value for CO2 is the sublimation point, to molecular weight, since molecules are held when dry ice, a solid, becomes a gas. a compound is the together by weak forces that are roughly proI then ask students to predict the boiling portional to mass. When the temperature is point of H2O. They admit that it should be temperature at which high enough to separate the individual molaround –200°C. Of course, we all know that a liquid becomes a gas. ecules from each other, they boil, or become water boils at +100°C. The hydrogen bonds a gas. between the water molecules raise its boiling I give students the following chart: point by approximately 300°C, thereby causing water to be a liquid at room temperature and making oceans, rivers, lakes, and, indeed, life possible. This year, my students commented that they knew everything I Molecular Weight Substance Boiling Point was telling them. They knew that N2, O2, and CO2 are all gases at room 2 H2 temperature. But they had never quite thought about it this way. H2O N2 O2 CO2 sUsan oFFner is a biology teacher at lexington high school, lexington, ma 02421; e-mail: soffner@ix.netcom.com. The American Biology Teacher, Vol. 74, no. 5, page 343. issn 0002-7685, electronic issn 1938-4211. ©2012 by national association of biology Teachers. all rights reserved. request permission to photocopy or reproduce article content at the University of california Press’s rights and Permissions Web site at www.ucpressjournals.com/reprintinfo.asp. doi: 10.1525/abt.2012.74.5.12 The american biology Teacher hydrogen bonds http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The American Biology Teacher University of California Press
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